Catalyst surfaces for the chromous/chromic redox couple

ABSTRACT

There is disclosed an electricity producing cell of the reduction-oxidation (REDOX) type divided into two compartments by a membrane, each compartment containing a solid inert electrode. A ferrous/ferric couple in a chloride solution serves as a cathode fluid which is circulated through one of the compartments to produce a positive electric potential disposed therein. A chromic/chromous couple in a chloride solution serves as an anode fluid which is circulated through the second compartment to produce a negative potential on an electrode disposed therein. The electrode is an electrically conductive, inert material plated with copper, silver or gold. A thin layer of lead plates onto the copper, silver or gold layer when the cell is being charged, the lead ions being available from lead chloride which has been added to the anode fluid. If the REDOX cell is then discharged, the current flows between the electrodes causing the lead to deplate from the negative electrode and the metal coating on the electrode will act as a catalyst to cause increased current density.

ORIGIN OF THE INVENTION

The invention described herein was made in the performance of work undera NASA contract and is subject to the provisions of Section 305 of theNational Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat.435, 42 USC 2457).

BACKGROUND OF THE INVENTION

Because of the energy crisis beginning in the mid-1970's and due toeconomic factors within the electric utility industry, there is a needfor storing bulk quantities of electrical power which might be producedintermittently or randomly by devices such as wind-driven generators,solar cells or the like. A number of methods have been consideredincluding the storage of compressed air in large reservoirs, flywheels,capacitive storage, inductive storage and a number of electric chemicalschemes. Electrochemical storage batteries are generally expensive,heavy and subject to deterioration when subjected to repeated charge anddischarge actions.

Up until now, only pumped water storage wherein water from a waterstorage pond at one level is directed to a water storage pond at a lowerlevel through a hydro-electric plant having a water pumping capabilityhas proven to be a viable method. Unfortunately, such facilities arelimited to areas where the terrain is suitable for providing watersources at different elevations.

Electrically rechargeable REDOX flow cell systems are well known andhave a very high overall energy efficiency as compared to other systems.Furthermore, REDOX type cells can be discharged more completely thansecondary battery systems. Additionally, REDOX cells are inexpensive ascompared to secondary batteries and do not deteriorate as significantlywhen repeatedly discharged in recharge.

DESCRIPTION OF THE PRIOR ART

As indicated previously, REDOX electrical cells are well-known. One ofthe best known REDOX cells uses an anode fluid having a chromic/chromouscouple and a cathode fluid having ferrous/ferric couple. In the priorart, electric potential from such a REDOX cell was obtained by terminalsconnected to respective inert electrodes, one being disposed in theanode fluid and the other being disposed in the cathode fluid. Carbonand graphite products which are inert to the anode and cathode fluidswere used as electrodes in some cases.

OBJECTIVES AND SUMMARY OF THE INVENTION

It is an object of the invention to provide a REDOX cell which willdeliver much greater current for any given electrode surface area thanprior art devices.

It is another object of the invention to provide a REDOX type cell whichwill deliver an increasingly greater percentage of current when comparedto prior art REDOX cells as the cell approaches a discharged condition.

Still another object of the invention is to provide a method of making aREDOX cell wherein certain desirable ions are provided in one of thefluids to produce a coating on its associated electrode as the REDOXcell is charged.

Yet another object of the invention is to provide a REDOX cell having acatalytic coating on one of the electrodes to enhance the activity ofthe desired ions in the one fluid.

In summary, there is provided a REDOX cell which may or may not have acatalytic coating on one electrode but including certain desired ions inthe fluid which contacts the electrode to coat it with the ions ascharging takes place.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of the REDOX cell embodying theinvention showing the anode and cathode fluid supply systemsschematically.

FIG. 2 is a graph illustrating the increased current density in a REDOXcell embodying the invention.

FIG. 3 is a graph illustrating current density versus potential fordifferent electrode materials.

FIG. 4 illustrates current density versus potential for a carbonelectrode with a catalytic coating and utilizing desired ions in thefluid to plate out on the coated electrode.

DESCRIPTION OF A PREFERRED EMBODIMENT

Referring now to FIG. 1, there is shown a REDOX cell 10 comprisingcontainer 11, divided into compartments 12 and 13 by an ion conductivemembrane 14. The graphite electrode 15 is disposed in the chamber 12 andconnected to an output terminal 6 while a graphite electrode 17 isdisposed in compartment 13 and connected to an output terminal 18.

In order to produce a voltage or potential difference between terminals6 and 18, a cathode fluid is passed through chamber 13 while an anodefluid is passed through chamber 12. As shown, cathode fluid from acathode fluid source 19 is circulated by a pump 30 through compartment13. Similarly, anode fluid from an anode fluid source 21 is circulatedby a pump 22 through the compartment 13.

The REDOX cell 10 utilizes an iron/chromium system wherein the cathodefluid contains ferrous/ferric couple while the anode fluid contains achromic/chromous couple. The anode fluid preferably contains water andHCl (aqueous solution of HCl) having dissolved therein a chromiumchloride salt. The cathode fluid likewise is water and HCl but hasdissolved therein an iron chloride salt. These fluids provide thedesired couples in each of the chambers 11 and 12. A more completediscussion of the couple, the fluid electrode requirements and membraneconsiderations is given in U.S. Pat. No. 3,996,064 which is incorporatedherein by reference. In accordance with the present invention, it hasbeen found that a coating of lead on the inert electrode 15substantially increases the current density of the electrode, andconsequently, the current available at terminals 6 and 18. The lead maybe coated onto the electrode 15 before it is disposed in chamber 12 tobe contacted by the circulating anode with fluid. It can also beobtained by dissolving lead chloride in the anode fluid before chargingthe REDOX cell.

When current is supplied to the REDOX cell 10 at terminals 6 and 18 tobring it up to a charged condition, a lead coating will deposit on theanode electroce 15. Conversely, as the REDOX cell is discharged bycurrent drawn from the terminals 6 and 18, the lead deplates from theelectrode and is redissolved into the anode fluid.

When the REDOX cell is being charged, chromium reduction occurs veryrapidly on the lead surface which is desirable for high current density.At the same time, the rate of hydrogen evolution is advantageouslyminimized. On a lead surface which was prepared by the electrodeposition of a very thin layer of lead onto a smooth carbon rod, thereduction current was measured to be 12 ma/cm² at an electrode potentialof -600 millivolts (mv) versus a standard calomel electrode (SCE). Underlike conditions, the reduction current for an untreated electrode wasonly 0.2 ma/cm². Thus, the lead coating on a carbon rod provided a60-fold increase of reduction current.

Referring now to FIG. 2, there is shown graphically the relativeperformance obtained over a wide range of polarizations of a solutionthat contained no lead ions and one that contained 10⁻⁴ molar of leadions.

Lead is representative of a class of non-noble metals that possess avery high hydrogen overvoltage and yet provides surfaces on whichchromic ion reduction is very rapid. Because lead is subject to anodicdissolution, it is plated onto the porous carbon or graphite electrodestructure during the recharging mode of the chromous/chromic reaction.Lead chloride is soluble in these acidified chloride solutions and thusremains in solution during the discharge mode of operations within aREDOX system. When the REDOX cell is being charged, the lead is platedout of the solution and back onto the carbon or graphite electrodestructure.

When the REDOX cell is being discharged, the lead functions very well asa surface for the reversible electrochemical oxidation of the chromousions to chromic ions. However, each time the REDOX cell is discharged,the lead undergoes electrochemical oxidation and is deplated from theanode surface and the activity for electrochemical oxidation of chromousion will then depend on the activity of the electrode structure itself.It has been found the carbon and graphite material are not very activefor this reaction.

In accordance with the invention, it has been found that silver, goldand copper are all electrochemically active as surfaces for the rapidelectrochemical oxidation of chromous ions, and at the same time, areless subject to being electrochemically oxidized than lead. FIG. 3graphically illustrates the current density versus potential for silver,copper and smooth carbon surfaces. As shown, at an electrode potentialof -550 mv versus a saturated calomel electrode the rate ofelectrochemical oxidation of a chromous ion is less than 0.1 ma/cm².Under the same conditions, the current silver surface is about 9 ma/cm²which is about 90-fold increase. A copper surface under the sameconditions yields 5 ma/cm² which is a 50-fold increase over a smoothcarbon surface.

Referring now to FIG. 1, there is shown a catalytic metal layer orcoating 16 on the inert electrode 15. The catalytic coating 16 isselected from the group of metals consisting of silver, gold and copperand may be applied to the electrode 15 by various procedures such aselectrodeposition, metal spraying, dipping or the like. Over thecatalytic layer 16 there is provided a layer of lead 17. The lead may becoated over the catalytic layer 16 before the electrode 15 is disposedin the chamber 12. As indicated previously, however, the lead layer 17may be plated onto the catalytic coating 16 by adding lead chloride tothe anode fluid before charging the REDOX cell.

Referring now to FIG. 4, there is graphically shown the current densityobtained for various electrode potentials for the chromous/chromiccouple where the anode electrode is coated with gold and wherein leadchloride is dissolved in the anode fluid to provide lead ions.

As indicated previously, the gold coating on the anode electrode permitsthe lead coating to deplate when the cell is being discharged without asignificant loss of electrode activity for the electrochemical oxidationof the chromous ion.

While the invention has been described with respect to the REDOX cellusing an anode fluid having a chromous/chromic couple and a cathodefluid having a ferrous/ferric couple, other couples may be used.

The anode fluid preferably contains water as a solvent having dissolvedtherein a chloride salt selected from the group consisting of titaniumchloride and chromium chloride, whereby cations in a reduced state areproduced. The cathode fluid preferably contains water as a solventhaving dissolved therein in a chloride salt selected from the groupconsisting of iron chloride, vanadium chloride, and manganese chloride,whereby cations in an oxidized state are produced.

Both the anode and cathode fluids are acidified solutions between 1 and4 molar.

Referring again to FIG. 4, with the gold coating on the anode electrodeit will be seen at negative 550 mv. versus the saturated calomelelectrode the current for the electro oxidation of chromous ion is 9ma/cm². This is a 90-fold increase over a plain carbon electrode.

The plating or coating of silver, gold or copper on the inert electrodeis very thin. The amount of metal in the coating is on the order of onlya few molecular layers. A graphite felt was produced with 25 microgramsof gold per cm² of projected area. This provided suitable electrodeperformance during discharge. The lead coating which goes over thesilver, gold or copper coating on the inert electrode, whether appliedbefore the electrode is inserted in the anode chamber or by depositionfrom the anode fluid, can also be as thin as several monolayers, where amonolayer is one molecule in thickness.

The gold, silver and copper catalysts which enhance the oxidation ofchromous ions during the discharge cycle may be dissolved in the anodesolution as salts to provide in-situ activation of the anode electrode.They can also be incorporated into the negative carbon electrode bysaturating the electrode with a salt solution of gold, silver or copperfollowed by heat treatment to dry it.

Although lead chloride salt has been discussed above, cadmium chloridecan be substituted for the lead chloride. Furthermore, cadmium chloridecan be used in combination with the lead chloride.

Cadmium chloride achieves the same results as lead chloride inincreasing the catalytic activity of a carbon electrode for thereduction of chromic chloride with the simultaneous inhibition ofhydrogen evolution.

It will be understood that changes and modifications may be made to theabove described invention by those skilled in the art without departingfrom the spirit and scope of the invention as set forth in the claimsappended hereto.

What is claimed is:
 1. A REDOX cell having first and second chambersseparated by an ion permeable membrane; an anode fluid in the firstchamber; anda cathode fluid in the second chamber; an anode electrode insaid first chamber, said electrode being electrically conductive butinert with respect to the anode fluid; a cathode electrode in saidsecond chamber, said electrode being electrically conductive but inertwith respect to the cathode fluid, said anode fluid having lead chloridesalt dissolved therein whereby lead is plated onto the anode electrodewhen the cell is initially charged.
 2. The REDOX cell of claim 1 whereinthe lead ions in said anode fluid are in the amount of 10⁻⁴ M/liter ofthe fluid to 10⁻⁵ M/liter.
 3. The REDOX cell of claim 1 wherein saidanode and cathode electrodes are selected from the group of materialsconsisting of carbon and graphite.
 4. The REDOX cell of claim 1 whereinsaid anode is coated with a thin layer of metal selected from the groupconsisting of silver, gold and copper to inhibit the oxidation of leadions when the cell is being discharged.
 5. The REDOX cell of claim 4wherein said thin layer of metal is 2-5 monolayers thick and amounts to10⁻⁴ to 10⁻⁵ M/liter of anode fluid.
 6. A REDOX cell having first andsecond chambers separated by an ion permeable membrane;an anode fluid inthe first chamber; a cathode fluid in the second chamber; an anodeelectrode in said first chamber, said electrode being electricallyconductive but inert with respect to the anode fluid; a cathodeelectrode in said second chamber, said electrode being electricallyconductive but inert with respect to the cathode fluid, said anodeelectrode having thereon a thin coating of lead.
 7. The REDOX cell ofclaim 1 wherein said coating of lead is about 2-5 monolayers thick. 8.The REDOX cell of claim 7 wherein the amount of lead is 10⁻⁴ to 10⁻⁵M/liter of anode fluid.
 9. The REDOX cell of claim 6 wherein a coatingselected from the group of metals consisting of silver, gold and copperis disposed between the anode electrode and the lead coating.
 10. TheREDOX cell of claim 4 wherein said coating disposed between the anodeelectrode and the lead coating is about 2-5 monolayers thick.
 11. AREDOX cell having first and second chambers separated by an ionpermeable membrane; an anode fluid in the first chamber; anda cathodefluid in the second chamber; an anode electrode in said first chamber,said electrode being electrically conductive but inert with respect tothe anode fluid; a cathode electrode in said second chamber, saidelectrode being electrically conductive but inert with respect to thecathode fluid, said anode fluid having cadmium salt dissolved thereinwhereby cadmium is plated onto the anode electrode when the cell isinitially charged.
 12. A REDOX cell having first and second chambersseparated by an ion permeable membrane;an anode fluid in the firstchamber; a cathode fluid in the second chamber; an anode electrode insaid first chamber, said electrode being electrically conductive butinert with respect to the anode fluid; a cathode electrode in saidsecond chamber, said electrode being electrically conductive but inertwith respect to the cathode fluid, said anode electrode having thereon athin coating of cadmium.
 13. The REDOX cell of claim 12 wherein saidcoating of cadmium is about 2-5 monolayers thick.
 14. A REDOX cellhaving first and second chambers separated by an ion permeable membrane;an anode fluid in the first chamber; anda cathode fluid in the secondchamber; an anode electrode in said first chamber, said electrode beingelectrically conductive but inert with respect to the anode fluid;acathode electrode in said second chamber, said electrode beingelectrically conductive but inert with respect to the cathode fluid,said anode fluid having dissolved therein more than one salt selectedfrom the group consisting of lead chloride and cadmium chloride.